doc(analytics): add README.md for GA4 analytics

[criticalAY]

Purpose / Description

Add documentation for analytics and the library related to it

Fixes

• Part of Fix analytics reporting (new property, new GA APIs) #14569

Approach

NA

How Has This Been Tested?

NA

Learning (optional, can help others)

NA

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

LGTM, thanks!